Seasonal Rainfall Outlook Platform
Historical skill · Jan 2027 · ONDJ 2026/27 — September initialization · ONDJ R3 (Deyr) rainfall domain

Historical skill · Jan 2027

How forecasts made with the same method scored in past years against climatology (the observed frequency of each tercile in the training years, about one-third each). The main number is the cross-validated 1993–2016 score: each year is forecast with parameters fitted without that year. Later years are exploratory: they were seen while the method was chosen.

Takeaway — Jan 2027

Historical skill is uncertain: cross-validated 1993–2016 RPSS is +0.002, and its interval (−0.013 to +0.016) includes no improvement over climatology. The exploratory 2017–2024 result (−0.007) is encouraging but those years were inspected during method selection.

Stronger historical evidence (interval above zero): ONDJ +0.133, Oct +0.091.

ONDJ 2026/27 (September initialization), ONDJ R3 (Deyr) rainfall domain: probability skill (RPSS) of the final method against climatology, with whole-year 95% intervals
TargetCross-validated 1993–2016 (main)In wordsExploratory 2017–2024
Jan+0.002 (−0.013 to +0.016)Small estimated improvement; skill uncertain−0.007 (−0.035 to +0.021)
ONDJ+0.133 (+0.032 to +0.245)Moderate improvement over climatology (interval above zero)+0.280 (+0.034 to +0.575)
Oct+0.091 (+0.025 to +0.159)Modest improvement over climatology (interval above zero)+0.103 (+0.005 to +0.241)
Nov+0.077 (−0.030 to +0.182)Small estimated improvement; skill uncertain+0.193 (−0.005 to +0.438)
Dec+0.002 (−0.044 to +0.051)Small estimated improvement; skill uncertain+0.043 (−0.020 to +0.098)

RPSS +0.05 means a 5% lower ranked probability score than climatology. An interval that includes zero means the gain is not established.

Technical detail: years better, p-values and blend weights
TargetYears better (cross-validated)p (one target / Holm)Years better (exploratory)Blend weight λ
ONDJ18 of 240.010 / 0.0406 of 80.18
Oct19 of 240.007 / 0.0376 of 80.41
Nov15 of 240.086 / 0.2575 of 80.32
Dec9 of 240.471 / 0.7995 of 80.70
Jan13 of 240.400 / 0.7994 of 80.90

Each interval and one-target p-value describes that target alone. Because 5 targets are examined, the Holm-adjusted p is the stricter test: ONDJ, Oct remain significant after adjustment. λ is the weight given to climatology in the blend (higher = closer to climatology).

Historical verification explorer

The evidence behind the scores above, for Jan 2027 in this area. Choose the evaluation period; all diagnostics use the blended probabilities against climatology.

Evaluation
Cross-validated (nested leave-one-year-out), 1993–2016
Benchmark
Climatology: observed tercile frequencies of the training years (about one-third each)
Area
ONDJ R3 (Deyr) rainfall domain; probabilities on 63% of it on average
Sample
24 years, 8,681 cell-years. Cells within a year are spatially correlated, so the independent sample is closer to one value per year.

Performance by year

Probability skill (RPSS) of each year; better than climatology in 13 of 24 years. Weakest years: 1993 (−0.074), 1994 (−0.062), 2013 (−0.041).

Better than climatologyWorse
−0.10−0.050.00+0.05+0.101993: RPSS −0.074 (forecast RPS 0.566, climatology 0.527; 364 cells)199319931994: RPSS −0.062 (forecast RPS 0.556, climatology 0.524; 364 cells)1995: RPSS +0.002 (forecast RPS 0.472, climatology 0.473; 364 cells)1996: RPSS −0.003 (forecast RPS 0.419, climatology 0.418; 364 cells)1997: RPSS +0.026 (forecast RPS 0.532, climatology 0.546; 320 cells)19971998: RPSS −0.023 (forecast RPS 0.404, climatology 0.395; 364 cells)1999: RPSS −0.003 (forecast RPS 0.408, climatology 0.407; 364 cells)2000: RPSS −0.011 (forecast RPS 0.433, climatology 0.428; 364 cells)2001: RPSS +0.033 (forecast RPS 0.477, climatology 0.493; 364 cells)20012002: RPSS +0.064 (forecast RPS 0.347, climatology 0.371; 364 cells)20022003: RPSS +0.015 (forecast RPS 0.536, climatology 0.545; 353 cells)2004: RPSS +0.050 (forecast RPS 0.467, climatology 0.492; 364 cells)2005: RPSS +0.010 (forecast RPS 0.353, climatology 0.357; 364 cells)20052006: RPSS −0.007 (forecast RPS 0.401, climatology 0.398; 364 cells)2007: RPSS +0.034 (forecast RPS 0.340, climatology 0.351; 364 cells)2008: RPSS +0.051 (forecast RPS 0.502, climatology 0.529; 364 cells)2009: RPSS +0.003 (forecast RPS 0.484, climatology 0.485; 364 cells)20092010: RPSS −0.021 (forecast RPS 0.435, climatology 0.426; 364 cells)2011: RPSS −0.022 (forecast RPS 0.529, climatology 0.518; 364 cells)2012: RPSS +0.046 (forecast RPS 0.444, climatology 0.465; 364 cells)2013: RPSS −0.041 (forecast RPS 0.523, climatology 0.502; 364 cells)20132014: RPSS −0.030 (forecast RPS 0.447, climatology 0.434; 364 cells)2015: RPSS +0.021 (forecast RPS 0.396, climatology 0.405; 364 cells)2016: RPSS +0.008 (forecast RPS 0.389, climatology 0.393; 364 cells)

Reliability

below normalnear normalabove normal
0.00.00.20.20.40.40.60.60.80.81.01.0below normal: forecast 33%, observed 35% (8681 cell-years)near normal: forecast 28%, observed 29% (1847 cell-years)near normal: forecast 33%, observed 31% (6834 cell-years)above normal: forecast 35%, observed 34% (7937 cell-years)above normal: forecast 40%, observed 41% (744 cell-years)Forecast probability

Points on the dashed diagonal are reliable: events forecast with probability p happen p of the time. Points flatter than the diagonal mean over-confident probabilities. Open points have fewer than 50 cell-years (0.5% of the sample, at least 50) and are not joined. Hover a point for its count.

Reliability table (forecast bins, observed frequency, counts)
CategoryBinMean forecastObserved frequencyCell-years
below normal30%–40%33%35%8,681
near normal20%–30%28%29%1,847
near normal30%–40%33%31%6,834
above normal30%–40%35%34%7,937
above normal40%–50%40%41%744

How strong are the signals?

Share of cell-years by the leading tercile probability.

weak (<40%)
92%
moderate (40–50%)
8%
strong (50–60%)
0%
very strong (≥60%)
0%
Category Brier scores (lower is better) and Brier skill against climatology
CategoryForecastClimatologyBrier skill
below normal0.22790.2274−0.002
near normal0.21220.2121−0.001
above normal0.22460.2260+0.006

Rainfall amounts

Historical rainfall-amount diagnostics (CRPS, CRPSS and amount errors) have not been generated for Jan 2027 in this area and period; this needs a separate cross-validated amount evaluation. Single-season amount scores are in the Verification section.

Historical performance by rainfall region

Probability skill by rainfall region (full regional matrix)
ONDJ 2026/27: probability skill (RPSS) by rainfall region, cross-validated 1993–2016, with whole-year 95% intervals and years better than climatology
RegionONDJOctNovDecJan
All Ethiopia+0.039 (−0.038 to +0.123) 14/24 yrs+0.013 (−0.038 to +0.064) 15/24 yrs+0.040 (−0.032 to +0.117) 14/24 yrs+0.003 (−0.026 to +0.031) 10/24 yrs+0.001 (−0.010 to +0.010) 12/24 yrs
R0 arid / marginal−0.059 (−0.207 to +0.071) 14/24 yrs−0.045 (−0.122 to +0.029) 10/24 yrs−0.095 (−0.213 to +0.022) 11/24 yrs+0.017 (−0.018 to +0.054) 12/24 yrs−0.002 (−0.010 to +0.006) 14/24 yrs
R1 western unimodal−0.003 (−0.089 to +0.081) 14/24 yrs−0.029 (−0.100 to +0.038) 14/24 yrs+0.044 (−0.003 to +0.095) 17/24 yrs−0.003 (−0.038 to +0.025) 14/24 yrs+0.001 (−0.010 to +0.011) 14/24 yrs
R2 Belg–Kiremt−0.033 (−0.121 to +0.063) 7/24 yrs−0.044 (−0.107 to +0.018) 9/24 yrs+0.009 (−0.062 to +0.090) 12/24 yrs+0.007 (−0.021 to +0.035) 12/24 yrs0.000 (−0.011 to +0.009) 13/24 yrs
R3 Gu–Deyr+0.133 (+0.032 to +0.245) 18/24 yrs+0.091 (+0.025 to +0.159) 19/24 yrs+0.077 (−0.030 to +0.182) 15/24 yrs+0.002 (−0.044 to +0.051) 9/24 yrs+0.002 (−0.013 to +0.016) 13/24 yrs
ONDJ R3 (Deyr) rainfall domain+0.133 (+0.032 to +0.245) 18/24 yrs+0.091 (+0.025 to +0.159) 19/24 yrs+0.077 (−0.030 to +0.182) 15/24 yrs+0.002 (−0.044 to +0.051) 9/24 yrs+0.002 (−0.013 to +0.016) 13/24 yrs

Positive values mean lower RPS than climatology. Intervals resample whole years; regions are the fixed 1993–2025 rainfall regimes, and the last rows are this cycle's rainfall domains. Regional results are noisier than national ones.